Enthalpies of Formation

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1 Enthalpies of FormationAn enthalpy of formation, Hf, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms.Standard enthalpies of formation, Hf, are measured under standard conditions (25°C and 1.00 atm pressure).Elemental source of oxygen is O2 and not O because O2 is the stable form of oxygen at 25 # and 1 atm, likewise with H2Elemental source of carbon is specified as graphite (and not, for example, diamond) because graphite is the lowest energy form of carbon at room temp and 1 atmWhy is the O2 stoichiometry left at "1/2"? The stoichiometry of formation reactions always indicates the formation of 1 mol of product. Thus, Hf values are reported as kJ / mole of the substance producedIf C(graphite) is the lowest energy form of carbon under standard conditions, then what is the Hf for C(graphite)?By definition, the standard enthalpy of formation of the most stable form of any element is zero because there is no formation reaction needed when the element is already in its standard stateHf C(graphite), H2 (g) and O2 (g) = 0

4 Chemical Bonds Three basic types of bonds: IonicElectrostatic attraction between ionsCovalentSharing of electronsMetallicMetal atoms bonded to several other atoms

5 Ionic Bonding Energetics of Ionic BondingAs we saw in the last chapter, it takes 495 kJ/mol to remove electrons from sodium.We get 349 kJ/mol back by giving electrons to chlorine.But these numbers don’t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic!

6 Energetics of Ionic BondingThere must be a third piece to the puzzle.What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion.Lattice Energy This third piece of the puzzle is the lattice energy: The energy required to completely separate a mole of a solid ionic compound into its gaseous ions. The energy associated with electrostatic interactions is governed by Coulomb’s law:Eel = Q1Q2d

7 Lattice EnergyLattice energy, then, increases with the charge on the ions.It also increases with decreasing size of ions.Which one of the following has the largest lattice energy?LiF, NaF, CaF2, AlF3LiCl, NaCl, CaCl2, Al2O3

11 Covalent Bonding In these bonds atoms share electrons.There are several electrostatic interactions in these bonds:Attractions between electrons and nucleiRepulsions between electronsRepulsions between nuclei

12 Electronegativity:The ability of atoms in a molecule to attract electrons to itself.On the periodic chart, electronegativity increases as you go……from left to right across a row.…from the bottom to the top of a column.

13 Polar Covalent BondsAlthough atoms often form compounds by sharing electrons, the electrons are not always shared equally.Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does.Therefore, the fluorine end of the molecule has more electron density than the hydrogen end.

14 Polar Covalent BondsWhen two atoms share electrons unequally, a bond dipole results.The dipole moment, , produced by two equal but opposite charges separated by a distance, r, is calculated:= QrIt is measured in debyes (D).The greater the difference in electronegativity, the more polar is the bond.

15 Lewis StructuresLewis structures represent molecules showing all electrons, bonding and nonbonding.Allows for two dimensional representation of molecular bonding.Shows the order of connectivityShows the types of bondsShows all electrons: bonding and non-bonding

16 Anatomy of Writing Lewis StructuresPCl3Find the sum of valence electrons of all atoms in the polyatomic ion or molecule.If it is an anion, add one electron for each negative charge.If it is a cation, subtract one electron for each positive charge.The central atom is the least electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds.Keep track of the electrons:26  6 = 20Fill the octets of the outer atoms.Keep track of the electrons:26  6 = 20  18 = 2Fill the octet of the central atom.Keep track of the electrons:26  6 = 20  18 = 2  2 = 0(7) = 26

17 Writing Lewis StructuresIf you run out of electrons before the central atom has an octet……form multiple bonds until it does.

18 Writing Lewis StructuresThen assign formal charges.For each atom, count the electrons in lone pairs and half the electrons it shares with other atoms.Subtract that from the number of valence electrons for that atom: The difference is its formal charge.The best Lewis structure……is the one with the fewest charges.…puts a negative charge on the most electronegative atom.

19 - Resonance + This is the Lewis structure we would draw for ozone, O3.But this is at odds with the true, observed structure of ozone, in which……both O—O bonds are the same length.…both outer oxygens have a charge of 1/2.-

20 ResonanceOne Lewis structure cannot accurately depict a molecule such as ozone.We use multiple structures, resonance structures, to describe the molecule.Just as green is a synthesis of blue and yellow…ozone is a synthesis of these two resonance structures.

21 ResonanceIn truth, the electrons that form the second C—O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygens and the carbon.They are not localized, but rather are delocalized.The organic compound benzene, C6H6, has two resonance structures.It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.

22 Exceptions to the Octet RuleThere are three types of ions or molecules that do not follow the octet rule:Ions or molecules with an odd number of electrons.Ions or molecules with less than an octet.Ions or molecules with more than eight valence electrons (an expanded octet).Odd Number of ElectronsThough relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of electrons.

23 Fewer Than Eight ElectronsConsider BF3:Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine.This would not be an accurate picture of the distribution of electrons in BF3.Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons.The lesson is: If filling the octet of the central atom results in a negative charge on the central atom and a positive charge on the more electronegative outer atom, don’t fill the octet of the central atom.

24 More Than Eight ElectronsThe only way PCl5 can exist is if phosphorus has 10 electrons around it.It is allowed to expand the octet of atoms on the 3rd row or below.Presumably d orbitals in these atoms participate in bonding.

25 More Than Eight ElectronsEven though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.This eliminates the charge on the phosphorus and the charge on one of the oxygens.The lesson is: When the central atom is on the 3rd row or below and expanding its octet eliminates some formal charges, do so.

26 Covalent Bond StrengthMost simply, the strength of a bond is measured by determining how much energy is required to break the bond.This is the bond enthalpy.The bond enthalpy for a Cl—Cl bond,D(Cl—Cl), is measured to be 242 kJ/mol.

28 Average Bond EnthalpiesThis table lists the average bond enthalpies for many different types of bonds.Average bond enthalpies are positive, because bond breaking is an endothermic process.NOTE: These are average bond enthalpies, not absolute bond enthalpies; the C—H bonds in methane, CH4, will be a bit different than theC—H bond in chloroform, CHCl3.

29 Enthalpies of ReactionYet another way to estimate H for a reaction is to compare the bond enthalpies of bonds broken to the bond enthalpies of the new bonds formed.Hrxn = (bond enthalpies of bonds broken)  (bond enthalpies of bonds formed)